First let me say that the sayings you are concerned about would not be copyrightable. Short sayings, or phrases are not entitled to copyright protection. Copyright requires that there be "works or authorship" in something like books, plays, poems, etc. The Copyright Office has never accept short sayings of the type you are concerned about as being able to be protected by the copyright law.
Rather, short sayings and phrases can be protected by trademark law, if they are used to identify the source of particular goods like shirts or other articles of clothing, or any other type of goods for that matter. Now how does one acquire ownership of a trademark in short sayings? It is basically in one of two ways: (1) by first use of the phrase in commerce on goods, or (2) by first registering the mark with the U.S. Patent & Trademark Office (or with your individual state's trademark office).
So, now the question is in your situation - who first used the phrase in commerce? You the shop owner or the person who came up with phrases. Well, it is really you who started using the phrases in commerce by physically putting the phrases on the shirts and selling them. Simply because someone asked you to put the phrase on a shirt does not, I believe, constitute using the phrase in commerce. So, I believe that you would have the better claim to ownership of a trademark in the phrases.
Now I mentioned that you can also acquire ownership to a trademark by first registering the phrase with the Trademark Office. You should check the Trademark Office's search tool to see if anyone had registered a phrase similar to the ones you are concerned about. You can do so by going here: Trademark Electronic Search System. I did a quick search myself, and I did not find these particular phrases trademarked. But I encourage you to do you own search on these phrases, as well as on any phrase which you think might be similar.
Then if you are serious about continuing to use these phrases in commerce, I recommend that you formally file an application to register a trademark with the Trademark Office. Let me give you the main Trademark Office page, where you can begin to understand the trademark application process: Trademark Application Process.
Let me also direct you to Trademark Basics. This will give you good information on understanding basic trademark law.
I hope this information helps. In summary I believe you would own a trademark to the phrases since you were the first to use them in commerce. Good luck.